You're replacing an Oxygen atom with a Sulfur atom. The original compound is Sodium Sulfate, which is Na2SO4, but when it becomes Sodium Thiosulfate, one of the Oxygen atoms is replaced with a Sulfur atom. So it becomes N2S2O3.

If it were to be Sodium Dithiosulfate, you would replace 2 Oxygen atoms with 2 sulfur atoms, so it would be N2S3O2